Linearity and dynamic range for complementary metal oxide semiconductor active pixel image sensors

ABSTRACT

A method and structure for a complementary metal oxide semiconductor active pixel sensor device having a photodetector, a sensing node electrically connected to the photodetector, an output connected to the photodetector, and a voltage-independent capacitance device connected between the sensing node and the output. The voltage-independent capacitance device provides a capacitance independently of a voltage on the sensing node. The voltage-independent capacitance device can be a voltage-independent capacitor, an electrode-electrode capacitor, or a common source amplifier and should have a capacitance larger than the capacitance of the sensing node. The voltage-independent capacitance device lowers an overall voltage-dependent capacitance of the APS.

FIELD OF THE INVENTION

[0001] The present invention generally relates to complementary metaloxide semiconductor (CMOS) active pixel sensors (APS) and moreparticularly to an improved pixel sensor that has increased linearity asa result of additional voltage-independent capacitance.

BACKGROUND OF THE INVENTION

[0002] CMOS APS are solid state imagers where each pixel contains aphoto-sensing means, reset means, charge conversion means, select means,and all or part of an amplifier. APS devices have the advantages ofsingle supply operation, lower power consumption, x-y addressability,image windowing, and the ability to effectively integrate signalprocessing electronics on-chip, when compared to CCD sensors.

[0003] In order to build high resolution, small pixel APS devices fordigital cameras it is necessary to use sub-μm CMOS processes in order tominimize the area of the pixel allocated the active components in eachpixel. In order to achieve good signal to noise performance it isimportant to hold as many photoelectrons as possible within the pixel.In typical APS pixel architectures the integrated photoelectrons areconverted to a voltage in each pixel. This charge to voltage conversionregion is typically a diode, either the photodiode or an isolatedfloating diffusion. It is the parasitic capacitance of the charge tovoltage conversion region that determines the maximum number ofelectrons that can be contained within the region. Sub-um CMOS processesare typically operated at low supply voltages, 3.3V and below, hence thereset level and the voltage swing that can be accommodated in the chargeto voltage conversion region is limited by the supply voltage. Since thesupply voltage is low, the signal swing on the charge to voltageconversion region is a large compared to the reset level. Since thecapacitance of the diode that forms the charge to voltage conversionregion is a function of the voltage across the diode, and the signalswing is large compared to the total voltage across the diode at reset,the capacitance of the diode changes substantially from the reset level,(or dark signal), to the saturation level, (or bright signal). Intypical APS pixel architectures the capacitance at reset is smaller thanthe capacitance at saturation. This produces a non-linear transferfunction. It is very important to have a linear transfer function forcolor image sensors. Non-linearity in the sensor response can degradethe color fidelity of the image. Response linearity has been optimizedfor CCD image sensors. APS are much less linear that CCD's.

[0004] In addition to poor linearity, APS sensors can also suffer fromlow charge capacity as a result of the reduced supply voltages in sub-μmCMOS processes. For the same pixel size, CMOS APS sensors have lowercharge capacity compared to CCD image sensors due to the larger supplyand clock voltages used on CCD image sensors.

[0005] One approach to providing an image sensor with the linearity of aCCD and the advantages of an APS device is to reduce the effect of thevoltage dependent capacitance of the charge to voltage conversion regionof an APS device. This invention does so by providing a voltageindependent capacitor in parallel with the diode capacitance of thecharge to voltage conversion region. This can also be used to improvethe charge capacity of an APS device.

SUMMARY OF THE INVENTION

[0006] It is, therefore, an object of the present invention to provide astructure and method for a complementary metal oxide semiconductoractive pixel sensor device having a photodetector, a charge to voltageconversion node, an amplifier input connected to the charge to voltageconversion node, and a voltage-independent capacitance connected inparallel with the charge to voltage conversion node. Thevoltage-independent capacitance provides a capacitance that is not afunction of charge placed on the charge to voltage conversion node. Thevoltage-independent capacitance can be an electrode-electrode capacitor,or the input capacitance of an amplifier.

[0007] The invention also comprises a method of manufacturing acomplementary metal oxide semiconductor active pixel sensor device whichincludes a photodetector, a charge to voltage conversion node, anamplifier input connected to the charge to voltage conversion node, anda voltage-independent capacitance connected in parallel with the chargeto voltage conversion node. The voltage-independent capacitance providesa capacitance that is not a function of charge placed on the charge tovoltage conversion node. The voltage-independent capacitance can be anelectrode-electrode capacitor, or the input capacitance of an amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The foregoing and other objects, aspects and advantages will bebetter understood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

[0009]FIG. 1 is a schematic diagram of an active pixel sensor pixel;

[0010]FIG. 2 is a schematic diagram of a second active pixel sensorpixel;

[0011]FIG. 3 is a graph illustrating the linearity of voltage output byactive pixel sensor pixels shown in FIGS. 1 and 2;

[0012]FIG. 4a is a schematic diagram of an active pixel sensor utilizinga voltage-independent capacitor,

[0013]FIG. 4b is a schematic diagram of an active pixel sensor utilizinga voltage-independent capacitor;

[0014]FIG. 4a is a schematic diagram of an active pixel sensor pixelutilizing a common source amplifier; and

[0015]FIG. 5b is a schematic diagram of an active pixel sensor pixelutilizing a common source amplifier.

DETAILED DESCRIPTION OF THE INVENTION

[0016]FIG. 1 is a schematic diagram that illustrates a CMOS APS pixel 5.As shown in FIG. 1, the cell 5 includes a photodiode 10, a transfertransistor 11 with a transfer gate TG, whose source is connected to thephotodiode, and a reset transistor 13 with a reset gate RG, whose drainis connected to the voltage supply VDD 14. The drains of the transfertransistor 11 and the source of the reset transistor 13 form a floatingdiffusion region FD 12 which functions as a charge to voltage conversionnode. The floating diffusion region 12 is connected to the gate of theinput transistor 15 of a source follower amplifier. The source of theinput transistor 15 is connected to the drain of the row selecttransistor 16, and the source of row select transistor 16 is connectedto the column bus 17.

[0017] Operation of active pixel sensor cell 5 is performed in threesteps: a reset step, where cell 10 is reset from the previousintegration cycle; an image integration step, where the light energy iscollected and converted into an electrical signal; and a signal readoutstep, where the signal is read out.

[0018] Referring to FIG. 1, during the reset step, the gate of resettransistor 13, and transfer transistor 11 is briefly pulsed with a resetvoltage (e.g. 3.3 volts). The reset voltage turns on reset transistor 13and transfer transistor 11 which pulls up the voltage on photodiode 10,and floating diffusion region 12 to an initial reset voltage.

[0019] Now the integration phase can commence. During integration, lightenergy, in the form of photons, strikes photodiode 10, thereby creatinga number of electron-hole pairs. Photodiode 10 is designed to limitrecombination between the newly formed electron-hole pairs. As a result,the photo-generated holes are attracted to the ground terminal ofphotodiode 10, while the photo-generated electrons are attracted to thepositive terminal of photodiode 10 where each additional electronreduces the voltage on photodiode 10. Thus, at the end of theintegration step, the potential on photodiode 10 will have been reducedto a final integration voltage where the amount of the reductionrepresents the intensity of the received light energy.

[0020] Following the image integration period, the readout periodcommences. First row select transistor 16 is turned on by applying aselect voltage, ( e.g. 3.3 volts) to the gate of row select transistor16. Next the gate of reset transistor 13, is briefly pulsed with a resetvoltage (e.g. 3.3 volts). The reset voltage turns on reset transistor 13which pulls up the voltage on floating diffusion 12 to an initial resetvoltage, typically less than or equal to VDD minus the reset transistorthreshold voltage. At this point the depletion region of the floatingdiffusion is at its maximum level and consequently the capacitance ofthe floating diffusion is at a minimum level. The floating diffusionreset voltage on the gate of source of source-follower transistor 15 isthen read out as a reset voltage level. Next the integratedphoto-electrons are transferred from the photodetector to the floatingdiffusion by pulsing the gate of transfer transistor 11. This reducesthe voltage on the floating diffusion 12. The floating diffusion signalvoltage on the gate of source-follower transistor 15 is then read out asa signal voltage level. The signal and reset levels are then subtractedproviding a voltage which represents the total charge collected by cell5.

[0021] The maximum number of photo-electrons or maximum signal leveltypically reduces the floating diffusion voltage level by an amount thatis all of, or a large percentage of the reset voltage on the floatingdiffusion. As a result, the floating diffusion depletion region widthchanges by a substantial amount compared to the initial depletion regionwidth after reset. This produces a variable floating diffusioncapacitance that is a function of the number of photo-electronstransferred to the floating diffusion. As the number of electronstransferred increases, the floating diffusion depletion region widthdecreases and the floating diffusion capacitance increases. Thisproduces a continuously non-linear transfer function.

[0022] The linearity problems created by voltage-dependent capacitanceare illustrated in FIG. 3. The vertical axis in FIG. 3 represents thevoltage of the floating diffusion region 12 while the horizontal axisrepresents the light level or integration time. The number ofphoto-electrons that are collected vs. light level or integration timeis a linear relationship. However, since the floating diffusioncapacitance increases as a function of the number of photo-electronscollected, the output signal provided to the column bus 17 from thefloating diffusion region vs. light level or integration time is not alinear relationship.

[0023] This relationship can be seen in the solid line A of FIG. 3. Morespecifically, line A represents a continuously non-linear transferfunction. This line has a continuously negative second derivative. LineA has a useable signal range 33 up to the voltage level Vsat′, basedupon a certain percentage deviation from a linear transfer function.This is can be much less than the total signal swing Vsat. While theamount of light energy (e.g., photons) received along the second portion30 of the response line can be calculated, such calculations can resultin higher noise in the rendered image. Therefore, for high image qualityapplications, the APS pixel output is only used for voltages along thefirst portion of 33 and not generally utilized for voltages above Vsat′.

[0024] This problem is more severe for the APS pixel shown in FIG. 2. Inthis case the photodiode also functions as the charge to voltageconversion node, and its diode capacitance comprises a much largerportion of the total capacitance associated with electrical node of thegate of the source follower input transistor. In this case the firstportion of the pixel response transfer curve 33 is much smaller thanthat for the case the case of the APS pixel shown in FIG. 1.

[0025] The invention mitigates these problems by reducing the percentageof the voltage-dependent capacitance compared with the total capacitanceassociated with the charge to voltage conversion node. Morespecifically, the invention reduces the percentage of thevoltage-dependent capacitance by including a larger voltage-independentcapacitance connected to the charge to voltage conversion node.

[0026] For example, in one embodiment, (shown in FIGS. 4a and 4 b), acapacitor C₁ 50 is connected to the charge to voltage conversion node12. The capacitor C₁ 50 is selected to have a very low voltagecoefficient to provide linearity and charge capacity for the reasonsstated above. More specifically, by adding additionalnon-voltage-dependent capacitance, the linearity and saturation voltageis increased. In a preferred embodiment the capacitor 50 comprises apolysilicon-polysilicon or other electrode-electrode capacitor. Suchcapacitors exhibit very low voltage coefficients and provide acapacitance that is independent of the voltage on the sensing node 12.

[0027] The dotted line B in FIG. 3 illustrates the pixel responsetransfer function achieved by adding a voltage-independent capacitancein parallel with the floating diffusion. The first portion of thetransfer function (portion 32) that does not deviate from a defmed levelof linearity, is increased compared to the prior art. Although the Vsathas decreased, since a fixed number of maximum electrons from thephotodetector are converted to a voltage by a larger capacitance, theuseful linear signal level Vsat′, and linear signal transfer function32, can be increased, while the second non-linear portion 31 isdecreased.

[0028] Further, with the inventive structure, the overall chargecapacity of the sensing node is increased, which is useful for caseswhere a large pixel and large photodetector are required.

[0029] Thus, as discussed above, with the inventive structure, thelinear signal response (e.g., portion 32) of the APS is dramaticallyincreased because the overall voltage dependency capacitance of the cellis reduced by adding voltage-independent capacitance device(s).

[0030] In addition, as would be known by one ordinarily skilled in theart, a combination of devices can be used to add voltage-independentcapacitance to the APS. For example, multiple capacitors 50 could beused to achieve the necessary level of capacitance.

[0031] In another embodiment the invention utilizes a common sourceamplifier 40 as the readout mechanism, rather than the source follower15 (e.g., see FIG. 5a and 5 b). The load for the common source amplifier40 is shown as item 41 along the column bus 17.

[0032] The input capacitance of a common source amplifier can be madelarger than that of a source follower amplifier by designing the commonsource amplifier voltage gain to be greater than 1. The inputcapacitance of the common source amplifier 40 is preferably larger thanthat of the source follower amplifier so that the sense node junctioncapacitance is a smaller component of the overall capacitance of thesense node to improve linearity, and so the total capacitance is largerto provide larger charge capacity on the sense node.

[0033] As would be known by one ordinarily skilled in the art given thisdisclosure, the input capacitance of the common source amplifier 40 canbe made (selected) larger by designing the common-source amplifiervoltage gain to provide the desired Miller effect on the gate-draincapacitance and the gate-channel capacitance of the pixel inputtransistor.

[0034] Additionally, a combination of the common source amplifier 40 andone or more capacitors 50 could be used to achieve the reduction in thepercentage of the of voltage-dependent capacitance of the sense node,and the corresponding increase in linear signal response discussedabove.

[0035] In addition the capacitor C, could be comprise a capacitance to anode other than ground, such as VDD.

[0036] Thus, the invention produces a greater linear signal response(e.g., portion 32) to light levels and has a higher voltage saturationlevel Vsat₂ because the voltage dependent capacitance of the cell isreduced by adding voltage-independent capacitance devices (40, 50).

[0037] While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims. PARTS LIST  5 CMOS active pixel sensor cell 10Photodiode 11 Transfer transistor 12 Floating diffusion region (FD) 13Reset transistor 14 Voltage supply VDD 15 Input transistor(source-follower) 16 Row select transistor 17 Column bus 30 Secondportion 31 Second portion 32 Transfer function (first portion) 33Useable signal range (first portion) 40 Common source amplifier 41 Item50 Capacitors A Solid line B Dotted line TG Transfer gate RG Reset gate

What is claimed is:
 1. An active pixel image sensor comprised of aplurality of pixels, at least one pixel comprising: a photodetector; atransistor; a charge to voltage conversion region coupled to saidphotodetector and connected to the input of said transistor; and acapacitor connected in parallel with the charge to voltage conversionregion wherein the capacitor is designed to have a low voltagecoefficient.
 2. The device in claim 1, wherein said capacitor provides acapacitance independently of a voltage on said charge to voltageconversion node.
 3. The device in claim 1, wherein said capacitorcomprises a polysilicon to polysilicon double plate capacitor.
 4. Thedevice in claim 1, wherein said capacitor comprises a polysilicon tometal interconnect double plate capacitor.
 5. The device in claim 1,wherein said capacitor comprises a metal interconnect to metalinterconnect double plate capacitor.
 6. An active pixel image sensorcomprised of a plurality of pixels, at least one pixel comprising: aphotodetector; a transistor; said photodetector also operating as acharge to voltage conversion region connected to the input of saidtransistor; and a capacitor connected in parallel with photodetectorwherein the capacitor is designed to have a low voltage coefficient. 7.The device in claim 6, wherein said capacitor provides a capacitanceindependently of a voltage on said charge to voltage conversion node. 8.The device in claim 6, wherein said capacitor comprises a polysilicon topolysilicon double plate capacitor.
 9. The device in claim 6, whereinsaid capacitor comprises a polysilicon to metal interconnect doubleplate capacitor.
 10. The device in claim 6, wherein said capacitorcomprises a metal interconnect to metal interconnect double platecapacitor.
 11. An active pixel image sensor comprised of a plurality ofpixels, at least one pixel comprising: a photodetector; a transistor; acharge to voltage conversion region coupled to said photodetector andconnected to the input of said transistor; and wherein said transistoris configured to operate as a common source amplifier.
 12. An activepixel image sensor comprised of a plurality of pixels, at least onepixel comprising: a photodetector; a transistor; a charge to voltageconversion region coupled to said photodetector and connected to theinput of said transistor; wherein said transistor is configured tooperate as a common source amplifier; and a capacitor connected inparallel with the charge to voltage conversion region wherein thecapacitor is designed to have a low voltage coefficient.